Import the subject audio:
aud = Import["http://home.ustc.edu.cn/~xiaozh/SE/del_silences.wav"]
silences = AudioIntervals[aud, #RMSAmplitude < 0.005 &];
Split the Audio object at the identified beginning and ends of the silence:
splitAuds = AudioSplit[aud, Flatten[silences]];
Lengthen the silences (by replacing them with silence of the correct length - there's no need to waste computation for AudioTimeStretch if we know we want silence):
lengthenedSilences = MapIndexed[If[OddQ[#2[]],
SampleRate -> AudioSampleRate[aud]],#1] &, splitAuds];
*Note that I'm using the nice alternating pattern of noise and silence to decide what to replace.
Now reassemble the Audio object:
result = AudioJoin[lengthenedSilences]
*You can of course play with the threshold used at the beginning to better define what "silence" is.